The present invention generally relates to inserts and methods for plugging or altering the orifice size of a through-hole, and more particularly steam balance holes in steam turbine wheels.
Rotor wheels of steam turbines are often equipped with balance holes through which steam leakage across the stationary nozzles of the turbine passes from stage to stage. The design intent of balance holes in an impulse stage design is to prevent leakage from reentering the main steam path through the turbine, avoiding disturbances in the main steam path that would lead to significant losses. The number and diameters of the balance holes are important, in that some of the leakage will reenter the main steam path if the aggregate cross-sectional area of the holes is insufficient for a given stage, while steam will be drawn from the main steam path into the leakage flow if the aggregate cross-sectional area is excessive for the stage.
Ongoing improvements in bucket, nozzle, and nozzle seal designs have reduced leakage flow, necessitating the use of fewer and/or smaller balance holes to maintain efficient operation of steam turbines. Because of the materials and costs involved in manufacturing steam turbine rotors (including their wheel and shafts), it is preferred to modify rather than replace rotors during retrofitting of a steam turbine. As disclosed in U.S. Pat. No. 7,134,841 to Montgomery, assigned to the assignee of the current application, a device can be installed in the steam balance holes of a steam turbine wheel to adjust and optimize the balance hole area during a steam turbine retrofit. While effective, further improvements would be desirable.